Tybody, I am a leader for AMC, Boston chapter bike trips. We have "beginner" level rides, where the average is 10-13 mph. Usually, it's around 10-11. Because of our liability rules, we have to have a sweep and we can't drop anyone, unless they tell us they are off the ride... not sure where south of Boston you live, but the Boston chapter and the SE MA chapter might have rides in your area.
Click on the chapter you're interested in and there should be a schedule.

http://www.outdoors.org/chapters/index.cfm